Probe microscopy such as scanning force microscopy and atomic force microscopy at the moment is expanding from use in pure qualitative analysis to making quantitative measurements.
How faithfully a scanning probe microscope shows the surface topography depends strongly on the size and shape of the ultrafine probe tip.
To allow the quantitative interpretation of the measuring results the probe tips used for measuring themselves have to be accurately measured before and after use in measuring, since the shape of the probe tips may change during the measuring procedure. If cone-shaped tips are used the tip diameter and the cone angle have to be accurately known. If flared or 2D tips are used the orthogonally oriented dimensions of the flat end of the probe must be accurately assessed.
In SPIE Vol.661 (1986), "Film Thickness and Refractive Index Standard Reference Material Calibrated by Ellipsometry and Profilometry" G. A. Candela et al. describe a standard reference material (SRM) consisting of a silicon wafer with a silicon dioxide film of uniform thickness. The silicon dioxide film contains windows used for stylus profilometry measurements with a mechanical depth very nearly the same as the oxide thickness. The depth, however, not being exactly the same as the oxide thickness due to native oxide forming in the windows affects the accuracy of the measurement.
In J.Appl.Phys. 74 (9), 1 Nov. 1993, "Dimensional Metrology with Scanning Probe Microscopes" J. E. Griffith and D. A. Grigg used an array of pillars with undercut sidewalls for probe characterization.
They found out that unfortunately the shape of the probe may change during a scan and that it is important to check the probe often. Frequently removing the probe for inspection, e.g. in a scanning electron microscope (SEM), however is impractical, especially if the scanning is performed in a controlled ambient or under vacuum. In most SEM inspection cycles the probe will be additionally contaminated and therefore its dimensions will be changed inherently.
The undercut of their probe characterizer serves two purposes: first it produces a cusp at the upper edge where the proximal point will hang as the probe scans over it and in addition the wall of the pillar is kept away from the probe, so that attractive forces between the probe and sidewall are kept to a minimum. To measure the total width of the probe and tip radius, the width of the pillar or hole scanned must be determined by other means.
As a result of their characterization efforts they state that calibration of a probe for use in steep topography requires a standard reference with steep topography and that in fabricating reference standards for optical linewidth measurements the uncertainty is determined by the roughness of the line edges.
To satisfy the gauge maker's rule which holds that measurements must be made to within 10% of the smallest dimension encountered they presume that calibration artifacts will have to be controlled at the atomic level. To carry that calibration over to the test structure it may be necessary to measure the probe dimensions at the atomic level too.
In their conclusion they point out that the biggest challenges in probe metrology are in the probe-sample interaction and in calibration, since in their opinion fabricating reference standards for calibration purposes poses some difficulties.
In "TopoMetrix, TI Standardizing AFM for Semiconductor Metrology", 38/Semiconductor International, October 1993, it is to be read that currently there are no industry-accepted standards for the calibration of AFM in applications such as Z-height measurement. According to this short article this limits the use of AFM for in-process metrology in lithography applications.
It is therefore an object of the present invention to provide calibration standards of high accuracy.
It is a further object of this invention is to measure probe dimensions and shape with great accuracy and to check scan linearity and accuracy.
It is an additional object of the invention to provide a method of fabricating the calibration standard.